The invention relates to an optically readable information disc comprising a substrate plate on one side of which there is present an information layer formed from a radiation cured lacquer. The information layer comprises an optically readable information track having information areas situated alternately at a higher and at a lower level along the track.
Such an information disc is known from Netherlands Patent Application No. 7.611.395 filed in the name of Applicants and corresponding to U.S. Pat. No. 4,275,091 issued June 23, 1981.
In the manufacture of this known disc, lacquers comprising mixtures of various acrylic acid esters (acrylates) are used. These multicomponent lacquers notably comprise a large quantity (60-90% by weight) of a monoacrylate in combination with 8-20% by weight of a diacrylate, triacrylate or tetraacrylate. In particular the combination of a large quantity of a monoacrylate with approximately 20% by weight of a diacrylate and/or a triacrylate is frequently employed. In Netherlands Patent Application No. 7.803.069 corresponding to U.S. patent application Ser. No. 006,609, filed Jan. 26, 1979 which relates to multilayer information discs, radiation curable lacquers, particularly ultraviolet light-curable lacquers are disclosed which comprise 50-80% by weight of a monoacrylate, 5-40% by weight of a di-, tri- or tetraacrylate, as well as 1-3% by weight of an initiator. In the examples of the last mentioned Netherlands Patent Application a multicomponent lacquer is described which comprises 78% by weight of 2-ethylhexylacrylate, 20% by weight of 1,1,1-trimethylolpropanetriacrylate and 2% by weight of benzoinisobutylether (initiator).
The use of multicomponent lacquers is feasible, because the lacquers have to satisfy a great number of often contradictory requirements. A person skilled in the art will endeavour to reach the desired diversity in lacquer properties by mixing various components lack of which have different physical and chemical properties and try to find a suitable compromise between the contradictory requirements.
As a result there exists a clear tendency towards the use of even more complicated multicomponent lacquers in applications having an extensive pattern of requirements.
In this connection it is to be noted that radiation-curable lacquers when used in information discs have to satisfy an extensive set of requirements. These include for example:
1. low viscosity, so that small layer thicknesses can be since the use of larger layer thickensses the shrinkage may give rise to deformation of the underlying substrate. PA1 2. rapid and good curing; PA1 3. good adhesion to the substrate plate; PA1 4. no adhesion to the metal matrix surface which is used in manufacturing the information disc; PA1 5. suitability for provision, after curing, with a reflective layer, for example a layer of silver or aluminum; PA1 6. optically transparently; PA1 7. after curing, non-deformability and chemically stability under varying climatological conditions.